1. Field of the Invention
The present invention relates to a method of analysis using an energy loss spectrometer and to a transmission electron microscope equipped with the energy loss spectrometer.
2. Description of Related Art
An electron energy loss spectrometer (EELS) is known as an apparatus for obtaining an energy loss spectrum of electrons transmitted through a sample by directing an electron beam at the sample. Electrons from the electron gun collide against the sample and thus suffer energy loss. Consequently, electrons with reduced energies are transmitted through the sample. The energy loss of the electrons transmitted through the sample varies depending on the species of the sample. Accordingly, the structure of the sample can be known by obtaining a spectrum of the energy loss.
In use, the EELS instrument is normally attached to a transmission electron microscope (TEM). FIG. 4 is a schematic view of the microscope. The microscope has an electron gun 1 for producing an electron beam which is focused by a condenser lens 2. A sample holder 3 is mounted behind the condenser lens 2.
An objective lens 4 focuses the electron beam transmitted through the sample placed on the sample holder 3. An intermediate lens 5 focuses the beam transmitted through the objective lens 4. A projector lens 6 magnifies the electron beam image transmitted through the intermediate lens 5. An observation chamber 7 is used to observe the electron beam image. An analyzer 8 receives the electron beam from the observation chamber 7 and obtains an energy loss spectrum. A lens system 9 focuses the beam transmitted through the analyzer 8. A detector 10 converts the electron beam image into an electrical signal. As an example, a CCD camera is used as the detector 10.
FIG. 5 is a schematic view of another transmission electron microscope. Like components are indicated by like reference numerals in both FIGS. 4 and 5. In this embodiment, the analyzer 8 is mounted between the intermediate lens 5 and the projector lens 6.
Especially, in a case where the analyzer is mounted in the imaging optical system, an operation for extracting and imaging only electrons with a certain energy can be performed, in addition to an operation for obtaining a spectrum. Therefore, there is an increasing demand for this kind of apparatus. Lenses are mounted behind the analyzer to magnify the image or spectrum.
The dispersive power of the analyzer at the energy dispersive surface is normally only several micrometers per volt. Energy dispersion referred to herein is an index indicating the distance by which electrons having energies differing by 1 volt are spaced apart. On the other hand, the resolution of the spectrum-recording medium is about 10 to 20 micrometers per pixel. The number of pixels is hundreds to thousands.
The maximum energy resolution required generally is less than about 0.5 eV. The maximum acquisition range of energies is more than hundreds of volts. To satisfy both requirements, it is necessary to highly magnify the energy loss spectrum produced at the energy dispersive surface of the analyzer or to suppress the magnification factor. That is, the magnification needs to be variable. Consequently, the lenses are disposed behind the analyzer.
The spectrum-recording medium acts also as a recording medium for electron microscope images. Generally, a two-dimensional CCD camera is used as the spectrum-recording medium. Electron energy loss spectroscopy is performed to obtain information about a sample by recording spectral images in the CCD camera 10, reading out the images, transferring them to a computer, and analyzing the distribution of the intensities of electron energies.
FIG. 6 shows the energy dispersive spectrum taken along the set direction of energy dispersion. The figure illustrates the manner in which the energy dispersive spectrum is taken along the set direction of energy dispersion. Shown in the figure are a spectral image A and an energy dispersive spectrum B. In the energy dispersive spectrum, electron energy (dE) is plotted on the horizontal axis. The distribution of the intensities of electrons having the same energy is shown in the spectrum.
A prior-art system of this kind for automatically setting the direction of energy dispersion taking account of rotation of the direction of energy dispersion according to variation of the magnification of the projector lens is known, for example, in Japanese Patent Laid-Open No. 2001-76664 (paragraphs 0008-0015 and FIG. 1)
Where a recently fabricated semiconductor device is investigated, if an elemental analysis is performed near a crystal grain boundary or electronic state of elements is investigated in relation to the dependence of the distance from the boundary, it is necessary to take a number of energy loss spectra while varying the electron beam position on the sample. The electron beam is directed at one point on the sample. The obtained spectrum is recorded in CCDs, read out, and analyzed.
At this time, it is important to minimize the spectrum acquisition time, for the following reason. If the acquisition time is prolonged, the sample will drift, the electron beam irradiation will damage the sample, or the sample will be more contaminated during this time interval. This will lead to a deterioration in the accuracy of analysis.
Even if the spectrum is stored in CCDs in a shorter time by increasing the electron density of the illuminating beam or enhancing the sensitivity of the CCD camera (e.g., in a time of 1/100 s), the read-in time will be as long as 4 seconds provided that the number of the CCD pixels is 2,000×2,000=4,000,000 and that A/D conversion and reading into the computer are done at 100 MHz. Thus, it is obvious that the read-in time is a decisive bottleneck.
This problem can be solved by making use of the function of CCD binning. FIG. 7 illustrates the CCD binning. It is assumed that there is a CCD unit having N×N pixels as shown. The sides of the CCD sensor area are taken in the X- and Y-directions, respectively. As an example, the pixels arranged in the Y-direction are combined into one. The whole pixel arrangement is represented by one-dimensional pixel array extending in the X-direction. The number of data items is reduced. In consequence, the operating speed can be improved dramatically. In the case of the example of FIG. 7, if the pixels arrayed in the Y-direction are all combined into one, and if electric charge is accumulated in the single linear array of elements extending in the X-direction, then N×N pixels are reduced to N×1 pixels.
Incidentally, an electron lens acts to rotate the image when the lens focuses it. Rotation of the spectrum due to the electron lens located behind the analyzer presents a problem. In the diagram of FIG. 6, the direction of energy dispersion has rotated to the right downwardly. An electron lens must be mounted behind the analyzer to vary the magnification factor. At this time, if the intensity of the lens is varied, the spectrum will be rotated.
Where the intensity of the spectrum recorded in CCDs is analyzed, the intensity must be projected in a direction perpendicular to the direction of energy dispersion, and the total must be taken. Accordingly, if the direction of energy dispersion of the spectrum is not parallel to any side of the CCD sensor area, it is impossible to utilize the CCD binning when images are read from the CCDs. Rather, all the pixels must be read out always. Depending on the direction of the spectrum, it suffices to extract some pixels rather than all the pixels. Yet, the efficiency is much inferior to the case where the binning mode is utilized.